Conventionally, some four-cycle engines mounted on a type of motorcycle, a scooter, have a cylinder axis line extending toward the front part of a vehicle, with an intake system connected to the upper face of a cylinder head, and with an exhaust pipe connected to its bottom face. A so called SOHC valve system is used in most of the engines of this kind, in which a single cam shaft is made to drive both an intake valve and an exhaust valve. In this type of valve system, the intake valve and the exhaust valve are connected respectively via a rocker arm to an intake valve cam and an exhaust valve cam formed on a cam shaft.
Besides the SOHC described above, there is a DOHC valve system used in a four-cycle engine mounted on other types of motorcycle, in which an intake valve and an exhaust valve are driven respectively by each individual cam shaft. A valve system of the DOHC engine is to be, in general, equipped with a valve lifter taking the shape of a bottomed cylinder at the end of the intake valve and at the end of the exhaust valve respectively, and a cam on an intake cam shaft or on an exhaust cam shaft is closely contacted with the top face of the valve lifter. The valve lifter is slidably fitted into a valve lifter guide hole in a cylinder head, and positioned along the axial line of the intake valve and the exhaust valve. In addition, a thin plate shim is interposed between the internal bottom face of the valve lifter and the intake valve or the exhaust valve for adjusting the gap between the top face and the circular base area of the cam (valve clearance).
It is desired to improve the driving performance of a scooter by adopting the DOHC valve system described above in the scooter engine.
However, when the DOHC engine is mounted on a scooter with its cylinder axial line extending toward the front part of the vehicle, the following problem may arise in trying to perform maintenance work while keeping the engine mounted on the vehicle body in the aforementioned manner and removing the cam shaft.
That is, in the mounted condition described above, each axial line of intake valves and exhaust valves, especially the exhaust valves located at the lower part, and of the valve lifter mounted at the end of the exhaust valve will extend downward to the front. Therefore, when the cam shaft is removed from the cylinder head in the abovementioned condition, no member is available for stopping an exhaust valve lifter, causing the valve lifter to fall off the cylinder head by its own weight. When the valve lifter falls off from the cylinder head, the shim interposed between the valve lifter and the exhaust valve will fall off as well. Since the shim is a smaller part compared with the valve lifter, it can be easily lost if it falls off the cylinder without being recognized by a worker.